Different planets could be cool. Few different ideas I've considered with planets (that I can remember):
1) Letting the player "travel" to a new planets after achieving a certain goal/mission. Effectively treating planets like "levels". Problem with this is that people seemed pretty attached to their base construction/builds, so there would have to be a good reason for having them abandon it and start over.
2) Starting on a visually different planets for each new game. Different biome/theme types, i.e. forest planet, ice planet, etc. Would give some visual variety to replaying the game.
3) Keeping the game on a single planet, but adding more biomes to the current world generation so that it's not all just Mars-esque dirt. Again, visual variety. Sometimes hard to get drastically different biomes to look good next to each other. Would allow different ways of spreading out resources that you need to search for.